When "git commit --amend" is used when no files are staged (for example,
to reword a commit message), git-secrets --pre_commit_hook scans the
entire working tree. This is because the scan() function is called with
an empty file list.

For large repos, this causes a bad user experience; the user expects
their editor to open immediately, but instead the pre-commit hook runs
for multiple seconds (9 seconds in the linux kernel on my workstation),
with no progress indication. If the repo contains any previously
committed files that match git-secrets patterns, the editor never opens,
and git-secrets' error message is displayed.

We can make the pre_commit_hook() function return immediately if there
are no changed files, which should match user expectations.

Closes awslabs#129